if your looking to watch this is real time, graph it etc.. You could use snmp to query specific interfaces and then graph it.. Cacti comes in handy here and is FREE.

More details of what you want exactly would help us point you how to accomplish what your after, are you trying to troubleshoot an issue? Just want to look at this for fun? Do you want to graph it? etc..

You could also turn on ip accounting and look at that for more details, etc. So knowing exactly what your looking for would be helpful